1. Field of the Invention
The present invention relates to an electro-mechanical brake with a differential gear, and more particularly, to an electro-mechanical brake with a differential gear that is attached to one side of a brake disc provided inside vehicle wheels to selectively reduce or stop rotations of the vehicle wheels along with an operation of a brake pedal by a user and in which when a driving unit that generates a rotational force by a power that is selectively applied along with the operation of the brake pedal by the user is driven, the rotational force of the driving unit is transmitted to a differential gear unit and an external braking unit and an internal braking unit that are connected to the differential gear unit simultaneously provide braking powers to the disc along with the rotation of the differential gear unit.
2. Description of the Related Art
In general, it is well known that a brake serves to decelerate or stop a vehicle by converting kinetic energy of a driving vehicle into heat energy by mechanical friction of a friction material to radiate the heat energy into the air.
In the driving method of generating the mechanical friction, a hydraulic caliper having a hydraulic cylinder and a piston is mostly used in the vehicle. A force by which a driver steps on a brake pedal is amplified through a hydraulic booster, and the amplified hydraulic pressure is transmitted to the hydraulic caliper (a large cross sectional area) attached to each wheel through an oil pipe and a master cylinder having a small diameter (a small cross sectional area).
As a result, the piston is pushed by a force obtained by multiplying the hydraulic pressure transmitted to a slave cylinder of the caliper by a cross sectional area of a caliper cylinder, and the piston moves a pad toward a disc to bring the pad into contact with the disc to generate a large clamping force.
A force obtained by multiplying the clamping force by a frictional coefficient of a contact surface between the pad and the disc is applied to the disc as a braking power. As a result, a small pedal force is converted into a high clamping force to be used for braking.
A hydraulic disc brake system that has been widely used is classified into a disc floating type, a fixed caliper type, and a floating caliper type.
First, since the disc floating type has a structure in which the disc is moved along with the movement of the piston while the caliper is fixed, noise may be caused in the disc, and reliability is poor. Thus, the disc floating type is rarely used.
Further, the fixed caliper type has a structure in which the pad is almost simultaneously clamped to the disc by two pistons on both sides of the disc.
Although the fixed caliper type has high reliability, since a volume is increased, a ventilation property may be poor.
Finally, the floating caliper type has a structure in which a hydraulic piston pressing unit moves an inner pad toward the disc to clamp the inner pad to the disc, a caliper housing slides in an opposite direction to the motion of the inner pad by a repulsive force after the inner pad is clamped against the disc, and an outer pad is pulled toward the disc to clamp and brake both side surfaces of the rotating disc.
Since the floating caliper type has a small number of components, a light weight and an excellent cooling function, the floating caliper type has been used in most vehicles. However, since the inner pad is first clamped in initial braking and the outer pad is operated by the repulsive force, there may occur a difference in wear between the inner pad and the outer pad.
Furthermore, a braking power at an initial brake operation may be insufficient until the outer pad is clamped.
Meanwhile, along with a change to a future vehicle such as a hybrid car, a fuel cell car and an electric car, safety of the vehicle, and a need for an environment-friendly vehicle, there is an increasing need for an electro-mechanical brake (EMB) in which an electric motor is applied to brake wheels instead of an existing hydraulic brake, so that various EMB products have been developed.
The EMB product has a structure in which a hydraulic driving unit (a hydraulic cylinder, a piston) that presses the pad is removed from the existing hydraulic disc brake to be replaced with a motor driving unit (motor, a roller screw, a decelerator), and the disc rotated using the motor as a driving source is braked.
Korean Patent Publication No. 10-2001-0032508 (filed on Apr. 25, 2001) discloses an electro-mechanical wheel brake for a vehicle including an electric motor; an operating device that is driven by the electric motor, converts a rotation driving motion of the electric motor into a linear motion, and has no auto locking function; and an electronic brake having a frictional brake pad that is pressed against a brake main body connected to vehicle wheels from the operating device through the driving of the operating device using the electric motor so as not to be rotated and a first electromagnet for operation. In the electro-mechanical wheel brake for a vehicle, the electric motor and the operating device are respectively fixed to their positions by using the electronic brake, and the electronic brake has a second electromagnet for operation.
Further, Korean Patent Publication No. 10-2009-0130602 (filed on Dec. 24, 2009) discloses a caliper-integrated electronic parking brake including an EPB-ECU that controls driving of a vehicle and receives a signal of an operation button to generate a control signal for parking; a driving assay that includes a decelerator in which an outer ring gear is circumscribed in parallel to a side surface of a gear wheel circumscribed in parallel to a side surface of a driving gear directly receiving a rotational force of a motor being rotated or reversely rotated under the control of the EPB ECU and has an axial force generating gear disposed to pass through the inner space of the outer ring gear in an axial direction to generate an axial-direction driving power for pressing the pad toward the wheel disc, and an integrated housing for accommodating the motor and the decelerator therein; a straight line motion unit that has a screw shaft receiving output torque generated in the driving assay to be moved in a straight line motion back and forth; and a caliper that is provided to surround a wheel disc attached to a vehicle wheel and implements a parking braking state of a vehicle by allowing a piston to press a pad by an axial-direction pressing force applied by a nut and the screw shaft as the straight line motion unit at the time of driving the motor to restrain the wheel disc.
The electro-mechanical parking brake (EPB) has a mechanism similar to the floating caliper type of the hydraulic disc brake.
That is, the EPB has a structure in which the inner pad is moved and clamped using the motor as the driving source and the sliding caliper is moved by the repulsive force after the inner pad is clamped to clamp the outer pad, so that the braking is performed.
The existing hydraulic disc brake and the electro-mechanical brake have basically the same mechanism except that the hydraulic pressure or the motor are respectively used as the driving sources in the existing hydraulic disc brake and the electro-mechanical brake, and the piston or the screw (gear) are respectively used as the driving power transmitting structure in the existing hydraulic disc brake and the electro-mechanical brake.
In other words, the inner pad is first moved and clamped, and the outer pad is clamped by the repulsive force to perform the braking.
In the two mechanisms, there may occur a different in wear between the inner pad and the outer pad in the floating caliper type. Further, it takes a longer time to reach a required braking power due to the operation principle (the caliper is moved to clamp the outer pad by the repulsive force after the inner pad is moved to be clamped) than the fixed caliper type, so that responsiveness may be degraded.
In order to obtain a braking power provided by the existing hydraulic pressure, a driving power transmitting manner such a ball screw method or a screw method having a relatively high deceleration ratio is mostly used in the EMB. Since the screw method has a driving power transmitting efficiency (efficiency: 0.6 to 0.8) lower than that (efficiency: 0.98) of a general spur gear or a helical gear, the screw method needs to select a driving motor of a higher output.
Moreover, since the ball screw method (efficiency: 0.9 or more) has relatively high manufacturing cost, unit cost of components may be increased.
In addition, in order to reduce the entire size of the EMB, the specifications of the motor need to be small. When the specifications of the motor are small, since a final output after gear shifting is performed is decreased, a braking power (1,200 Kgf) for maintaining parking braking and provision of the vehicle and during driving is decreased. For this reason, reliability for performance of the EMB may be degraded.
In addition, when a motor driving power is large, the braking power for maintaining the parking braking is sufficiently generated. However, since the entire size and weight of the EMB are increased due to the motor, the weight of the vehicle may be increased. Accordingly, the attachment may be easily degraded.
Meanwhile, a relatively simple mechanism in which the driving power of the motor is transmitted to a rack and a pinion without a differential gear may be designed.
However, in this case, when the wears of the inner pad and the outer pad are constantly generated, the two pads can be simultaneously clamped.
This is because strokes of the inner pad and the outer pad (distance by which racks are moved from a driving power transmitting shaft) are constantly the same.
That is, when there is a difference in wear between the inner pad and the outer pad, only one pad is first clamped, and a certain portion of the firstly clamped pad is worn. Thereafter, both of the inner pad and the outer pad are clamped. Accordingly, in this case, accurate responsiveness may not be expected.